Ad Hoc Networks
Ad hoc wireless relay networks have a number of advantages over conventional structured networks, such as cellular networks. Ad-hoc wireless networks do not require a fixed infrastructure. This can reduce cost. Because signals can he transmitted via different routes, reliability is increased. Energy consumption can decrease because intermediate relay nodes can receive and retransmit the signals via shorter hops. Because the total transmitted energy is decreased, the lifetime of the network is increased, there is less interference, and spectral efficiency is improved.
As a disadvantage of ad hoc relay networks, a route from a source node to a destination, node needs to be specified. The route specifies a sequence of nodes involved in relaying of the information, a transmission protocol, energy allocations at the nodes, and a transmission schedule.
In conventional ad-hoc relay networks, the source node typically encodes information with an error-correcting code, and the information is transmitted via the sequence of nodes in the route. The encoded information is called a codeword. As used herein, a codeword can be a file or block of data bytes.
Rateless Codes
In situations where the channel state information (CSI) is unknown, a weak block error-correcting code can result in unreliable communication due to noise, while a strong block error-correcting code wastes energy. Rateless codes solve this problem. Instead of encoding the information as a predetermined number of bits using a block error-correcting code, the information is encoded into a potentially infinite stream of bits. Then, the receiver only decodes bits in the flow until the information is recovered.
Rateless codes include fountain codes, punctured low-rate block codes, Reed-Solomon codes, convolutional codes and turbo-codes. Rateless codes were originally designed for erasure channels. However, rateless codes also work well for additive white Gaussian noise (AWGN) channels. Rateless codes have also been used for Ethernet, point-to-point, broadcast and multicast applications.
Routing Protocols
A relay node receives a signal from a previous node. The node can either amplify the signal in an amplify-and-forward protocol, or the node can decode the information in the signal, re-encode the information, and then retransmit the signal in a decode and-forward protocol to the next node. This process continues until the signal is received by the destination node. Such a relaying scheme is known as multi-hop routing. Conventionally, only one node is forwarding the signal to a next node at any one time using a single route with multiple sequential hops from the source node to the destination node.
A number of methods are known for selecting optimal routes in multi-hop ad hoc relay networks. In one method called “flooding,” each node broadcasts the information along with the address of the node and the energy (or power) used for the transmission. The destination node can then select the optimal route from the accumulated information it receives. The nodes participating in the route can then be informed of the route using feedback information.
If all channel state information is available at a single location, then, the optimal route can be determined using an optimization technique that minimizes the total energy consumption in the network, or other optimization criteria. Distributed methods for optimal, route determination are also known.
Another issue in ad-hoc networks is flow control. With flow control, the source node transmits a continuous bit stream (flow), and the relay nodes direct the flow concurrently over several, parallel links to the destination node. Relay nodes can receive multiple such flows, and re-distribute the flows to subsequent relays in the network until the flows reach the destination node.
Cooperative Relay Networks
Cooperative relay networks provide an alternative for single route multi-hopping. In a cooperative relay network, multiple nodes can concurrently transmit the information. In a conventional (state-of-the-art) cooperative relay network, multiple nodes transmit the same information, and a receiving node accumulates the energy from all of the signals before decoding the information. This can increase the diversity order at the receiving node. In addition, the information can he transmitted concurrently over several parallel routes for route diversity. Thus, a probability of deep fades is decreased. Furthermore, the accumulation of the energy increases the energy efficiency of the transmission.
Networks that use cooperative communications have a different set of problems such as parallel route selection, and code selection for particular links. Expected energy cost under the assumption of cooperation and network topology are also issues.
Prior art route selection and optimization techniques generally assume that the nodes along the route perform energy accumulation. However, routes selected in that manner are sub-optimal when used in networks where the nodes accumulate mutual-information. Therefore, the prior art routing methods are not applicable for networks according to the invention.
The routing protocols described in the commonly assigned parent application Ser. No. 11/377,711, “Cooperative Relay Networks using Rateless Codes” filed by Molisch et al. on Mar. 16, 2006, assume that any node that has sufficient mutual information participates in the transmission. In a quasi-synchronous protocol, a specific scheduling scheme is assumed, and only two hops are allowed between source and destination. Obviously, a two hop network is relatively restrictive.
An asynchronous scheme makes special assumptions about the starting time of a node transmission and the duration, and allocates transmit power equally.
it is desired to proved route selection in a more general manner for networks that are unconstrained in size.